A series of experiments are being conducted to determine the relative cariogenicity of the different serotypes of S. mutans and certain S. mutans strains of special interest. These experiments employ monoinfected gnotobiotic rats maintained on a standardized test regimen. Experiments are scheduled sequentially as only one or two isolators are available at a time. Results to date suggest that reproducible differences do exist in the cariogenicity of different strains. Additional experiments are under way to determine the parameters of these differences.